1. Field of the Invention
This invention relates to a system and method for orientation of the mirrors located along the closed optical path of a ring laser gyroscope. More particularly, this invention is directed to a mirror orientation system and method for a ring laser gyroscope in which a resultant speckle pattern from each mirror is formed by directing a coherent light beam into the optical path, and reflecting the light off the surface of each mirror in the path. The particular speckle pattern associated with each mirror surface is then isolated. An optimum mirror beam location along the reflecting surface of each mirror with respect to the laser beam and an optimum rotational angle of the mirror about its normal is determined through photo-detection methods such that the speckle pattern is minimized. The mirrors are then secured to the ring laser gyroscope in the position along the optical path, providing superior mirror positioning with reduced total resonant retroscatter.
2. Description of the Related Art
Ring Laser Gyroscopes form a modern alternative class of optical rotational sensors, providing inertial guidance information by other than mechanical gyroscopic systems. Ring laser gyroscopes are designed to measure the phase shift induced by rotation between at least two oppositely directed light beams traversing a closed optical path which encloses an area. This phase shift, which arises during rotation of the closed path, is used to measure the amount of gyroscopic rotation. The rotation-induced phase shift is known as the Sagnac effect, and is due to the fact that during gyroscope rotation, the distance traversed by a clockwise light beam may be different than the distance traversed by a counter-clockwise light beam over the same optical path, providing the "effective plane" of the path is not parallel to the axis of rotation.
If ring laser gyroscopes were ideal devices, the rate at which light beam wave phase shifts (measured by detecting fringes) at a detector would be directly proportional to the rate of rotation. Ideally, if the ring optical path of the gyroscope were not rotating, the light beam wave fringes would be static and fixed with respect to the frame to which the mirrors are attached.
Practical ring laser gyroscopes are subject to bias and frequency locking problems which can cause errors in the phase shift information as detected "rotation", when the gyroscope is actually stationary, or vice versa. Practical ring laser gyroscopes are active laser devices, where a gas laser occupies a portion of the closed optical path. Gas flow in a preferred direction, such as cathode to anode in a D.C. discharge region, gives rise to bias. One way to correct this bias is by providing two anodes, symmetrically positioned about the optical path, to offset flow effects.
Frequency locking, known in the art as "lock-in", is a problem that is prominent at low rotation rates. Practical closed optical paths for ring laser gyroscopes are defined by a plurality of spaced apart mirrors. Minute imperfections in the reflecting surfaces of the mirrors gives rise to the "lock-in" phenomenon. The imperfections can cause a small fraction of incident light to backscatter. Coupling between the counter-directed light incident and the backscattered light can give rise to the standing wave characteristic of "lock-in".
Heretofore, the avoidance or reduction of lock-in due to backscatter has been addressed by mechanical or optical biasing schemes, directed to an operating gyroscope during gyroscope operation. Examples of these schemes include: a body dithered ring laser gyroscope, as in U.S. Pat. No. 4,115,004 to Hutchings (issued to the assignee of this application); mirror servoing, as in U.S. Pat. Nos. 4,152,071 to Podgorski and 4,422,762 to Hutchings et. al. (762 issued to the assignee of this application); mirror dithering, as in U.S. Pat. No. 4,281,930 to Hutchings (issued to the assignee of this application); and, optical mode locking reduction schemes, as in U.S. Pat. No. 4,522,496 to Sanders (issued to the assignee of this application). More recently, U.S. Pat. Nos. 4,641,970 to Gustafson et. al., 4,657,392 to Egli, and 4,695,160 to Egli, all are directed to lock-in correction, discriminating, or error cancelling systems during gyroscopic operation. All the foregoing systems operate from a premise that certain levels of backscatter will occur and, therefore, the only way to address errors that arise as a result of lock-in is during laser gyroscope operation.
The lock-in of the foregoing gyros can be a major error source. The only attempt made at the present time to control the scatter that causes lock-in is to use very low scatter mirrors and position them so no obvious macroscopic defects are impinged upon by the laser beam. These designs hence attempt to reduce the level of backscatter, by means of translation of the mirrors with respect to the laser beam, at the time of gyroscope manufacture.
It has been known, heretofore, that scattering from highly polished optical surfaces are due to microirregularities of only a few nanometers or less. See, J. M. Elson, et. al., Scattering from Optical Surfaces, in Vol. VIII, Chapter 7, APPLIED OPTICS AND OPTICAL ENGINEERING, P. 191-244 (1979, Academic Press, Inc.), for a complete discussion of this type of scatter. This article notes that both scalar and vector theory may be applied to the analysis of scattering where the surface roughness is less than or near light wave length. Such scattering is not easily described by geometric optics.
One type of scatter phenomenon that arises in the presence of coherent light is laser speckle. Speckle may be thought of as a self-interference phenomenon between light waves coming from different elementary areas of a rough surface having height variations on the order of a wavelength of light. See, Robert K. Erf, APPLICATION OF LASER SPECKLE TO MEASUREMENT, in Laser Applications, Volume 4, pp. 1-69 (Academic Press, 1980). In particular, laser photography has been used to measure the out-of-plane rotation or tilt of a rough surface. Also, studies have been made to investigate the laws which govern spatial movement of laser speckles; F. P. Chiang, et. al., Laws of Laser Speckle Movement in Space, OPTICAL ENGINEERING, Vol 5 (25), pp. 667-670 (May 1986).
Heretofore, it was known that when laser light is incident on a finely irregular surface, the fine structure of the surface (which may even be small with respect to the wavelength of light) imparts a random phase modulation to the reflected light. (See, Karl A. Stetson, A Review of Speckle Photography and Interferometry, in OPTICAL ENGINEERING, VOL. 14 (5), September, October, 1975).
As light propagates away from this rough surface, the speckle effect may be observed in an image plane removed from the surface, using a lens. This speckle field at any point in space may be thought of as the sum of components arriving from a variety of directions. The amplitude of field components may be close to one another, but their relative phase, although static, will be random. These properties of the speckle pattern of light reflected from a rough surface were known as a means for measuring in-plane deformation. However, none of the speckle pattern research has heretofore been applied to ring laser gyroscope technology.
A method for alignment to minimize losses, optimize mode structure and hence maximize the desired resonance of the laser device is assumed to be used; e.g., "Alignment of Resonant Optical Cavities," by Dana Z. Anderson, Applied Optics, Vol. 23, (Sept. 1, 1984). This commonly consists of tangential translation of the mirrors for maximized output power. This can be done passively (on an optical resonance) or actively as a laser.